A long-recognized important objective in the constant advancement of digital IC (Integrated Circuit) technology is faster speed and lower power dissipation. For example, a digital system such as a modem microprocessor is comprised of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) operating at a low rail-to-rail voltage of 1.5V. In addition, at such a low operating voltage, the gate oxide is designed to be in the range of tens of angstroms (.ANG.). Such a thin gate oxide is advantageous for faster switching speed of the MOSFETs.
Furthermore, the digital system typically must interface with other electronic systems which operate at higher voltages. For example, typical communications systems operate at a rail-to-rail voltage of 3.3V. However, a thin gate oxide of the MOSFET in the digital system may be adversely affected by coupling with the higher operating voltage of an external system. The gate oxide may break down if the gate is coupled to a high operating voltage or the operating life time of a MOSFET having the thin gate oxide may be diminished if the gate is coupled to a high operating voltage.
Nevertheless, an output buffer, within a digital system having MOSFETs with thin gate oxides, interfaces the digital system to an external system with higher operating voltage. The output buffer drives an output, which couples to the external system, to the higher operating voltage. However, since the output buffer is comprised of MOSFETs having thin gate oxide, a mechanism is desired for protecting the MOSFETs in the output buffer while the output buffer drives the output to a higher operating voltage of the external system.